"We are talking about the origin of galaxies and stars and even life itself."

The 7.5m Herschel probe, named after the astronomer William Herschel the discoverer of Uranus, will be the largest telescope ever put into space, built around a huge 3.5m diameter mirror.

Its high-tech sensors, which detect far-infrared radiation, are designed to peer through dust clouds to observe the mysterious process of how stars and galaxies are created.

Meanwhile Planck, named after the father of quantum mechanics, will study the minute fluctuations in the background 'echo' left over from the Big Bang - called Cosmic Microwave Background - to map the universe in the finest detail ever.

Scientists say the results, which should give clues to the structure of the infant universe 14 billion years ago, could radically change human understanding of the most fundamental principles of physics.

Professor Richard Holdaway, of funding body the Science and Technology Facilities Council, said: "Planck is going to allow us to travel right back to the very beginning of time."

Cambridge University physicist Professor George Efstathiou, leading member of the Planck team, said there were many different theories about the crucial 'inflation' period immediately after the Big Bang, but much was effectively guesswork without vital experiments like this.

A crucial question is how the intricate structure of today's universe could have evolved from a concentrated point of pure energy following the Big Bang.

The professor said: "The physics of the Big Bang is still largely unknown.

"We know there must have been structure there at the time of the Big Bang, and possibly before the Big Bang.

"That's what we hope this will tell us."

The results could also offer insights into the much vaunted string theory - science's big hope for a unified theory of everything. The idea involves a complex 11-dimensional universe, with seven 'hidden' dimensions on top of the four observable dimensions of space and time.

Professor Efstathiou said: "The potential for fundamental new discoveries that will change our understanding of physics is very important and that is what I'm really hoping for with Planck.

"Unravelling the physical information may tell us something about the warped geometry of the hidden dimensions."

Cardiff University Professor Matt Griffin on the Herschel team said very little was known about the insides of dust clouds where stars are born because traditional telescopes cannot see in.

He said: "We are only just beginning to scratch the surface of this can tell us.

"This launch will mark the end of a long and gruelling era for those who developed Hershel, but also the start of an exciting new era when we begin using Herschel to unveil the secrets of the universe."

The two satellites have been 10 years in the planning and development, with contributions from a dozen countries. The total cost of the projects is estimated at between 1.5 and 2 billion euros.

Professor Southwood, the ESA director of science and robotic exploration, admitted he would have a "few sleepless nights" before lift-off from the Kourou Spaceport in French Guiana.

He said: "It's like the birth of a new baby and I won't feel comfortable until you we know it is all working properly in two or three months.

"We only get one shot at launch and there's no chance to fix it later.

"But this is a magnificent piece of European technology with the UK playing a leading and very important role in that. It's well worth the money."